How do you know that "the artist" slowed the songs down? The question is "slower than what" or "slower than whom?" Do you mean slower than a particular recording? Many songs exist in multiple versions at many tempi. Varying tempo is part of interpretation.

As a singer, I like to linger a bit. As a guitarist, I'm not up to uptempo work, so I don't do it. In my case slowness is sometimes a preference and sometimes a necessity. I don't like to be rushed in singing a song, but neither do I like to be dragged down by a chorus of loud singers dragging out the ends of my lines -- sometimes groups slow or rush a tempo, especially with a capella material